The present invention relates to a packet switching exchange system for private branch exchanges which, during comparatively low traffic of users packet calls, makes possible the packet switching for Integrated Services Digital Network (ISDN) terminal equipment on the D-channel between the terminal equipment and an ISDN, and is particularly concerned with one which, in the case of a private branch exchange separated into a circuit interface and a circuit switching-system control unit, allows the operation of packet switching without hampering the circuit switching performance of the circuit switching-system control unit.
What is called ISDN (acronym for integrated services digital network) provides diversified services with or without the use of telephones in a unified manner on the basis of the digital transmission of information. The feasibility of the integrated services largely depends upon implementing a multipurpose user network interface which can be shared by various services using telephones or no telephones. Such a user network interface may be provided in the International Telegraph and Telephone Consultative Committee (CCITT) recommendations I.410 to I.464, but now there are various interface constructions specified such as, principally, the basic interface and the primary rate interface.
The basic interface and the primary rate interface have such formats on the transmission line as are illustrated in FIGS. 4(a) and 4(b) respectively. The channel construction for the basic interface is 2B+D, namely the number of information channels (B) 2 and that of signal channels (D) 1. The channel construction for the primary rate interface is, for instance, 23B+D, namely the number of information channels 23 and that of signal channels 1. In general, an information channel (hereinafter B-channel) is offered to users for circuit switching communication and packet switching communication, the segment between users and the network being circuit switching communication. A signal channel (hereafter D-channel), on the other hand, is used for packet type transmission of the B-channel distribution control information (which is called "control packet") needed for implementing the communication on the B-channel. The communication between users and the network is carried out by packet switching, and user information of packet type (which is called "user packet") can be directly transmitted. Thus, packet switching is also possible with the D-channel. In packet switching, as shown in the CCITT recommendation X.25, information is divided into blocks of a certain size, and each block is transmitted and switched in a state (called "packet") where destination and control information are added to it. Unlike circuit switching, packets from a terminal are temporarily stored in the exchange, their destination is judged by the exchange and the packets are transferred at high speed to the destination through the network.
The user packet on the D-channel has such a transmission format conforming to the CCITT recommendations I.441 and I.462 as illustrated in FIG. 5.
The user packet is composed of data divided into blocks of a certain size, a packet header including the receiving terminal's address necessary for the connection up to the terminal, a number for controlling terminal information for each physical circuit between the ISDN and a terminal accommodated by it and for identifying terminals set up by multipoint distribution and information indicating that it is a user packet. Therefore, despite the physical connection by only one circuit between the ISDN and each terminal, when each terminal set by multipoint distribution sends user packets randomly to more than one receiving station at the same time, each user packet is transmitted after sorted out one by one within the network according to the destination information added to it, with the result that packet transmission is possible with any receiving station. Likewise, packet transmission to any receiving stations is possible on the B-channel by using a single circuit for connection to the ISDN and by sending packets including their destination information from a terminal to receiving terminals.
As understood from above, it is known that within an ISDN user packets can be transmitted by circuit switching on the B-channel and by packet switching on the D-channel as well. Concerning packet switching on the D-channel, there have been such papers as Packet switch configuration in the ISDN switching system (SSE88-76), Society for Telecommunications in Japan pp. 71-75 and network system for ISDN NTT Research and Implementation Report Vol. 36, No. 36 (1987) pp. 967-975. In the D-channel processing section, according to those papers, user packets and control packets related to the B-channel distribution control information are identified and separated. Then user packets are multiplexed and concentrated and are sent along with the B-channel user packet by a packet handler to packet switching equipment where they are processed by packet switching. Therefore, processors in ISDN exchanges usable for packet data are required to identify and separate all user packet calls and control packets. Moreover, the central control unit in a circuit switching system cannot be free from restriction caused by the priority of telephone calls which demand immediate processing. This hinders the use of the same processors for controlling packet data, so the construction of the central control unit is split into packet switching processors connected to a high speed bus and circuit switching processors controlling switching operation and resources. Another difficulty lies in that it is necessary between packet switching processors and packet handlers to transmit at high speed the resource information for the connection of user packet calls.
Assuming the circuit switching method or the packet switching method in which a private branch exchange for circuit switching performs switching between each extension terminal and an ISDN, it has been necessary to use hardware and software special to processing packet calls in addition to those special to circuit switching calls. Moreover, since D-channel and B-channel user packets separated by circuit switching equipment are multiplexed by a packet handler and processed by packet switching equipment, it has been necessary to transmit massive data at high speed. This has been the cause for a limited throughput of a whole system for packet calls.